Rubber is made in an emulsion or wet process in water or in a solution process in an organic solvent. Various monomers can be used and are polymerized in a free-radical polymerization process for emulsion rubber or an anionic polymerization process for solution rubber. In a typical application for the emulsion process, styrene and butadiene monomers are mixed together in water and additives including a modifier, an emulsifier and an activator are added to the solution to form a feed stream. The feed stream is fed to a heat exchanger that removes heat from the feed stream. An initiator is added, and the feed stream with the initiator flows through a series of stirred reactors. Polymerization occurs as the material flows through the reactors and continues as long as styrene and butadiene monomer units are available in the solution. To stop the polymerization at a desired polymer chain length, a short-stopping agent, such as hydroquinone, is added. The reactor product stream flows into a blowdown tank, and steam is added to strip out styrene and butadiene monomer. An aqueous latex is collected in tanks. A coagulating agent is added to the tanks, and a rubber crumb is formed, recovered, dried and baled for shipment to a tire or other manufacturer of final rubber products.
In a typical application for the solution process, styrene and butadiene monomers are initiated by alkyl lithium compounds in an organic solvent and are polymerized via anionic polymerization. A nitrogen blanket is typically required for the polymerization process. Randomization agents are usually added to produce a random copolymer. The microstructure of the copolymer, including vinyl content, molecular weight distribution and end-chain or in-chain functionalization, may be fine-tuned under different reaction conditions. After the polymerization process, the polymer solution is flash-distilled to form a more concentrated solution, which is then distilled with steam to remove the organic solvent and yield a rubber crumb. Dispersants and coagulants are generally added in the aqueous phase to control the size of the crumb. The crumb may be further stripped by steam and then filtered, dried and baled for sale.
In making tires and other rubber products, it is desirable to mix silica with an elastomer or rubber to improve certain properties of the elastomer. It is well known to incorporate silica into rubber using a dry mixing process, where a material is put on the surface of the silica during the mixing process to allow it to blend into the rubber. When the silica is coated with such an agent, the silica is referred to as hydrophobated and any material used to make hydrophobated silica is a hydrophobating agent. Silane compounds have been developed as hydrophobation agents. Processes for incorporating silica into rubber using the dry mixing process have been effective, but are time-consuming and energy-intensive. In the dry mixing process, rubber, silica, a silane and some of the compounding ingredients are fed into a Banbury rubber mixer, where the silica reacts with the silane to form a reaction mix compound. This pass may take 10 minutes or longer and greatly reduces the efficiency of the mixing operation. The mixing process requires a great deal of time, capital and operating and maintenance expense. Further, ethanol from the silane must be removed in the mixing step or in a downstream processing step. Rubber mixing facilities are not designed to function as chemical plants, and extra equipment must be installed to either isolate or to burn the alcohol so as to meet environmental standards. The reaction mix compound is further mixed in a remill step, where additional ethanol may be removed and additional compound ingredients may be added. This pass, however, is used primarily to improve the dispersion of the silica and reduce the Mooney viscosity of the stock. The remilled stock is combined with curatives in a final mix to produce a rubber compound suitable for use in tire tread stock.
Rather than mixing silica into rubber after the rubber has been made, silica can be added to the process in which rubber is made to make a silica-rubber masterbatch. One problem in making a silica-rubber masterbatch is when untreated silica is added to an emulsion of styrene-butadiene rubber or SBR (the emulsion or wet process), or to a solution of SBR in an organic solvent (the solution process), the silica does not completely incorporate into the polymer and separates as fines when coagulated. These fines not only reduce the value of the masterbatch, but also cause a processing problem in that the fines have to be disposed of or recycled.
Silica-filled rubber masterbatch has been made successfully by incorporating the silica into rubber as the rubber is made in an emulsion process. Silica can be hydrophobated efficiently in an aqueous setting with common silane compounds. In the tire industry, silane compounds that contain sulfur are commonly used because the sulfur provides active sites for coupling the silane compound to the polymer chains in the mixing steps. The dispersion of silica in rubber is strongly dependent on the degree of hydrophobation of the silica. The dispersion of the silica in the polymer can strongly influence the final properties of the rubber compound, such as wet traction, rolling resistance, and tread wear. It is often advantageous to make a silica-rubber masterbatch and dilute it down to a desired level of silica during mixing with rubber that does not contain silica, and thus avoid poor performance of a final rubber product due to a poor dispersion of silica in the rubber, which can occur in dry mixing for a variety of reasons.
U.S. Pat. No. 8,357,733, issued to Wallen et al., discloses a process for hydrophobating silica and a process for making a silica-filled-rubber masterbatch using rubber made in an emulsion process. The silica was hydrophobated to make it compatible with the rubber, and a hydrophobation agent was used that binds to rubber in a vulcanization process for incorporating the silica into a final rubber product, particularly a tire. U.S. Patent Application Pub. Nos. 20120322925, 20130203914 and 20130203915 disclose a process for hydrophobating silica and for making silica-rubber masterbatch with various elastomers in aqueous conditions. The hydrophobation process of the silica is performed with a silane compound, and the hydrophobated silica is mixed into a latex and incorporated into rubber during coagulation in an emulsion process.
Rubber product manufacturers, including tire manufacturers, prefer to incorporate some rubber made using a solution process into their products for particular properties that solution-made rubber imparts in the final product, particularly in tires. Although it has not been possible, or at least not economical, to make a silica-rubber masterbatch similar to the one disclosed in the U.S. Pat. No. 8,357,733 using rubber made in a solution process, where the rubber is made in an organic solvent, efforts have been made to provide a mixture of silica and solution-made rubber. U.S. Pat. No. 7,307,121, issued to Zhang, treats silica in an organic solvent with a mercaptosilane and a silane coupling agent such as bis(triethoxysilylpropyl) disulfide (TESPT). The treated silica is blended into an organic solution of SBR, and a masterbatch of solution-made rubber and silica is recovered by steam stripping to remove solvent. U.S. Pat. No. 6,025,415, issued to Scholl, describes a process for the production of mixtures of solution rubbers and surface-modified oxidic or siliceous fillers. Solution rubbers were mixed with surface-treated silica in solvent, and both were added dropwise into hot water and steam to form a uniformly distributed silica mix. U.S. Pat. No. 6,713,534, issued to Goerl et al., describes a process for making a fine-particle rubber powder, which included suspending one or more silicatic fillers and one or more bifunctional organosilicon compounds or a silicatic filler modified with an organosilicon compound in water to obtain a suspension, which was adjusted to a pH of 5 to 10. Rubber, which was made by the solution and/or the emulsion process, was dissolved in an organic solvent, and the resulting solution was added to the suspension. The organic solvent was removed to obtain a rubber powder in water, and the water was removed to obtain the fine-particle rubber powder. U.S. Pat. No. 7,790,798, issued to Chen, describes a method for making a silica-rubber masterbatch that contained a diene elastomer and silica in an organic solvent. An unhydrophobized precipitated silica having a first average particle size was mixed and ground in a first organic solvent, and a ground silica slurry having a reduced second average particle size was formed. The ground silica slurry was mixed with a diene elastomer in a second organic solvent; and the solvent was removed to form a solution masterbatch preparation.
A silica-filled-rubber masterbatch has not been commercialized that uses rubber made in a solution process, possibly because the incompatibility between inorganic silica and rubber made in an organic solvent has not been overcome satisfactorily. Consequently, there remains a need for a silica-filled-rubber masterbatch that contains rubber made using a solution process, where the masterbatch can be economically manufactured and where the silica is hydrophobated in a manner such that the silica will become sufficiently homogeneously mixed into the rubber and bound to the rubber during curing to produce a vulcanizate having acceptable properties.